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C nanospheres as anode for long-life sodium-ion batteries
Accepted Manuscript Nitrogen Doped Hollow MoS2/C Nanospheres as Anode for Long-life SodiumIon Batteries Yangsheng Cai, Hulin Yang, Jiang Zhou, Zhigao Luo, Guozhao Fang, Sainan Liu, Anqiang Pan, Shuquan Liang PII: DOI: Reference:
S1385-8947(17)31097-5 http://dx.doi.org/10.1016/j.cej.2017.06.146 CEJ 17227
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
12 April 2017 9 June 2017 25 June 2017
Please cite this article as: Y. Cai, H. Yang, J. Zhou, Z. Luo, G. Fang, S. Liu, A. Pan, S. Liang, Nitrogen Doped Hollow MoS2/C Nanospheres as Anode for Long-life Sodium-Ion Batteries, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.06.146
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Nitrogen Doped Hollow MoS2/C Nanospheres as Anode for Long-life Sodium-Ion Batteries Yangsheng Caia,b, Hulin Yanga,b, Jiang Zhoua,b,*, Zhigao Luoa,b, Guozhao Fanga,b, Sainan Liua,b, Anqiang Pana,b,*, and Shuquan Lianga,b,* a
School of Materials Science and Engineering, Central South University, Changsha 410083, P.
R. China b
Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of
Education, Central South University, Changsha 410083, P. R. China
Abstract As a complement or alternative to lithium ion batteries, development of highperformance sodium ion batteries with long cycle life is urgent. Here, nitrogen doped urchinlike MoS2/C hollow structures are successfully synthesized by using Mo-MOFs as the precursors with a facile sulfuration process. The nanocomposites that contain carbon and nitrogen elements possess good electronic conductivity as well as expanded interlayer, which are beneficial for electron transportation and ion diffusion. The hierarchical hollow structure is able to accommodate volume change during discharge/charge. When applied as anode for sodium-ion batteries, the N-doped MoS2/C anode exhibits good rate capability and outstanding cycling performance. It delivers a high initial discharge capacity of 972 mA h g -1 at 100 mA g−1. The discharge capacity can achieve 242 mA h g-1 even at 5000 mA g-1. Importantly, long-term cycling test shows that the capacity maintains a considerable capacity of 128 mA h g-1 after 5000 cycles at 2 A g-1, with a capacity fading rate of 0.036% per cycle. Keywords: molybdenum disulfide, metal organic frameworks, hollow structure, nitrogen doped, sodium-ion batteries
Corresponding author: Tel.: +86 0731-88836069. Fax: +86 0731-88876692. E-mail address: [email protected] (J. Zhou), [email protected] (A. Pan), [email protected] (S. Liang)
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1. Introduction Sodium-ion batteries (SIBs) are gradually regarded as a promising candidate for largescale energy storage system due to the abundant Na element in nature and almost the same electrochemical mechanism to lithium-ion batteries (LIBs) [1-4]. Actually, due to the inherent large ionic radius (RNa=1.02 Å) and heavy relative atomic mass (uNa=22.9898), SIBs usually exhibit unsatisfactory electrochemical performance, especially the low specific capacity and poor cycling stability [5-7]. Therefore, it is considerable to develop desirable electrode materials for SIBs. Molybdenum disulfide (MoS2) with a large interlayer spacing (d=0.62 nm), exhibiting a reversible capacity as high as 670 mA h g−1, may be an attractive anode materials for sodium storage [8-11]. Particularly, the 2D layered structure in MoS2 is constructed by the stack of three atomic layers (S−Mo−S) through Van der Waals interactions, which is beneficial to the intercalation/de-intercalation of Na+ ions [12]. However, the practical application of bare MoS2 as SIBs is suffered from the low electronic conductivity and great volume variation [8, 9, 13]. It is a validated strategy to overcome above problem by decorating MoS2 with carbon-based materials [8, 9, 14-22]. For example, the novel MoS2/carbon-nanotubes delivered a remarkable reversible capacity (461 mA h g-1 at 500 mA g-1) as anode material for sodium-ion cell [22]. Shi et al. assembled MoS2/C hierarchical nanotubes, exhibiting remarkable rate behavior as SIBs anodes [8]. The MoS2/C microspheres also manifest an amazing long-life performance of 390 mA h g−1 at 1000 mA g−1 after 2500 cycles [9]. Nevertheless, there is still an opportunity to enhance MoS2 based anode materials with excellent cycling stability at high-rate capability. The architecture of metal ions and organic linkers, metal organic frameworks (MOFs), have become increasingly fashionable on account of novel structure, chemical stability and promising carbonaceous ramification [23-28]. The pioneering work on the use of MOFs as template/precursor for the synthesis of porous carbon has been reported by Xu et al. in 2008 [29]. Zheng et al. fabricated an N-doped carbon materials derived from ZIF-8, which 2
displayed the enhanced ability of ion diffusion and electron transportation for energy storage [30]. Specially, the self-templated carbon and nitrogen source derived from MOFs are considered as desirable conductive complements to improve the electrochemical performance of transition metal-based compounds [29-34]. Zhu et al. synthesised a Co9S8/carbon composite using MOFs as precursor, which exhibited outstanding electrocatalytic performance [34]. It is a popular application of the electrode materials for energy storage applications, especially the metal oxides and sulfides (selenides), which transformed from MOFs [33-37]. Our group has derived various MOFs to porous metal oxide [35], ternary oxide [36], hybrid bimetallic metal oxides [37] and so on, which all exhibit excellent electrochemical performance. To the best of our knowledge, there is no report concerning about the self-templated synthesis of MoS2 nanostructures by the use of Mo-MOFs. In this work, we report the synthesis of nitrogen doped MoS2/C hollow structures with expanded interlayers by sulfuration of Mo-MOFs. The formation mechanism of the urchinlike morphology is investigated. Benefit from the composited carbon and nitrogen, the present MoS2 is supposed to possess high conductivity. As a proof, the MoS2/C nanocomposites are employed as anode materials for SIBs, which exhibit good rate capability and excellent longterm cycling performance.
2. Experimental section 2.1 Materials Synthesis. Molybdenum-based MOFs was synthesized according to the previous literature [38]. Typically, 3.5 g MoO3 and 1.66 g imidazole were mixed in a 500 mL round-bottom flask. Then 250 mL deionized (DI) water was added under vigorous stirring overnight by a reflux device. The white powders, Mo-MOFs, were collected by suction filtration with DI water and dried in vacuum at 50 °C for 12 hours.
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To obtain hollow N-doped MoS2/C nanospheres, the Mo-MOFs were treated via a sulfuration process. Briefly, 100 mg Mo-MOFs, 100 mg glucose and 400 mg thiocarbamide were dispersed in 20 mL DI water by sonication for 15 min. Then, the suspension was poured into a 50 mL Teflon-lined stainless-steel autoclave and reacted at 200 ℃ for 12 h. The black precipitate (as-prepared MoS2/C) was collected by suction filtration with DI water and ethanol, and dried in a vacuum overnight at 50 ℃. To obtain the final product (annealed MoS2/C), the black precipitate was heated under Ar atmosphere at 700 ℃ for 2 hours with a heating rate of 5 min-1. For comparison, pure MoS2 were prepared by using a similar method, except that the Mo-MOFs was replaced by the Na2MoO4, and the glucose was not added to the reaction. 2.2 Material characterization The as-synthesized Mo-MOFs and MoS2/C were characterized using X-ray diffractometer (XRD, Rigaku D/max 2500, Cu), field-emission scanning electron microscopeenergy dispersive X-ray analysis (FESEM-EDS, FEI Nova Nano-SEM 230), transmission electron microscopy (TEM, JEOL-JEM-2100F). The valence state of the element in MoS2/C was determined by an X-ray photoelectron spectra analyses (XPS, ESCALAB 250Xi). The BET surface area and pore distribution of MoS 2/C were measured by a Quantachrome Instruments (NOVA 4200e). The content of carbon, nitrogen and sulfur in the samples have been tested by carbon-sulfur analyzer (CS-444) and nitrogen-oxygen analyzer (TC-436), respectively. 2.3. Electrochemical measurements The electrochemical properties of were conducted in CR 2016 coin cells assembled in a professional glove box (Mbraun, Garching, Germany). The cathode electrodes were prepared by dispersing MoS2/C (70 wt.%), acetylene black (20 wt.%) and polyvinylidene fluoride binder (10 wt.%) in N-methyl-2-pyrrolidone solution as slurry, which were next coated on copper foils and dried in a vacuum oven at 100℃ for 12 h. The sodium metal plates and glass 4
fiber were performed as counter electrodes and separator. A commercial solution containing 1 M NaClO4 and ethylene carbonate/dimethyl carbonate (EC/DMC; 1:1, v/v) was used as electrolyte.
The
measurements
of
cyclic
voltammetry
(CV)
and
galvanostatic
charge/discharge were finished by utilizing the CHI604E electrochemical workstation (Chenhua, Shanghai, China) and Land battery tester (Land CT 2001A, China) with the testing voltage range from 0.01 to 3 V versus Na+/Na.
3. Results and Discussion The urchin-like MoS2 nanocomposites were synthesized by a facile sulfuration approach used Mo-MOF as precursors (Scheme 1). As shown in Fig. 1a, the XRD peaks of assynthesized Mo-MOFs is in accord well with previous investigation [39], confirming the successful preparation of Mo-MOFs. After the treatment of sulfuration, the X-ray diffraction pattern (shown in Fig. 1b) of the as-obtained black powders can be indexed to hexagonal 2HMoS2 phase (space group: P63/mmc (194), a = 3.16116 Å, b = 3.16116 Å, c = 12.2985 Å, JCPDS card No. 37-1492). As shown in the XRD pattern of as-prepared MoS2/C (after hydrothermal reaction), there are two peaks at 2θ=33.2 and 57.1° that can be indexed to (100) and (110) planes of hexagonal 2H-MoS2. After post-annealing at 700 ℃, the (002) diffraction peak at 2θ=13.44° appears, and the intensity of other peaks is also reinforced, demonstrating the increased crystallinity of annealed MoS2/C. Notably, the (002) peak was shift toward low angles compared with the standard doubling angles (14.38°), indicating the expanded interlayer distance from 6.15 to 6.49 Å. The expanded interlayer may be due to the incorporation of foreign atoms or molecules, such as oxygen atom or NH 4+ [11, 40, 41], which would be advantageous for Na+ ion diffusion. The morphology of the products was observed by TEM. As shown in Fig. 2a, the annealed MoS2/C presents a uniform morphology of urchin-like nanospheres with hollow interior. The magnified TEM image shown in Fig. 2b further confirmed the hierarchical 5
structures, whose external and inner diameter are about 400 and 200 nm, respectively. According to the previous reports, the hollow nanospheres structure is beneficial for battery application, as this structure can accommodate the volume change during discharge-charge process [17, 42]. From an enlarged observation of the edge (Fig. 2c), there are numberless stacked 2D MoS2 nanosheets and amorphous carbon layer is coated the nanosheets. The lattice fringes with spacing of 0.656, 0.287 and 0.234 nm in the high-resolution TEM image (Fig. 2c-d) are clearly marked, which can be indexed to (002), (100) and (103) crystal faces, respectively. The nitrogen adsorption/desorption analysis was executed to acquire the surface information of the annealed MoS2/C. A type-IV isotherm (Fig. 3a) indicates that the annealed MoS2/C possesses mesoporous structure. The BET specific surface area of the annealed MoS2/C is as high as 51.04 m2 g-1, which could be beneficial for SIBs applications. The BJH curve (Fig. 3b) illustrates a sharp peak at 3.8 nm, confirming the presence of mesoporous channels in the urchin-like hollow MoS2/C architectures. The XPS analysis was employed to investigate the chemical state of the as-prepared MoS2/C and annealed MoS2/C nanocomposites. Fig. S1a (in Electronic Supporting Information (ESI†)) depicts the Mo core level XPS spectra of the as-prepared MoS2/C. There are two pairs of peaks (the one are at 231.8 and 228.5 eV, the other are at 233.2 and 229.6 eV), corresponding to 1T-MoS2 and 2H-MoS2, respectively [43]. A small peak at 225.7 eV is related to S 2s [8, 14]. As shown in the Mo core level XPS spectra (Fig. 4a), there is only a pair of distinct peaks at 232.9 eV for Mo 3d3/2 and 229.7 eV for Mo 3d5/2, identifying with the 2H-MoS2 phase [8, 44]. The disappearance of characteristic peak for 1T-MoS2 indicated that the metastable phase was transformed to 2H-MoS2 after thermal treatment. The peak at low binding energy (236.0 eV) is attributed to oxidation state of Mo (Mo6+), which results from the surface oxidation of MoS2 [44, 45]. In addition, the S 2p core level spectra of both asprepared MoS2/C and urchin-like MoS2/C (Fig. S1b and Fig. 4b) present a couple of peaks, 6
corresponding to S 2p1/2 and S 2p3/2, demonstrating the presence of S2- ions [14, 15]. As shown in Fig. S1c and Fig. 4c, the similar C 1s spectra of as-prepared MoS2/C and annealed MoS2/C can be spilt up into three peaks at 284.6 (284.8), 285.9 (286.0) and 287.8 (287.0) eV, which can be ascribed to C–C, C–N and C–O bond [46, 47]. Furthermore, the Raman spectra (Fig. S2) also verified the existence of carbon species in the as-prepared MoS2/C and annealed MoS2/C. Both of them present a pair of prominent peaks at approximately 1345 and 1592 cm1
, which are associated with disordered carbon (D-band) and graphitic carbon (G-band),
respectively. The results indicate that carbonization of the glucose and the Mo-MOFs have occurred during the sulfuration process under hydrothermal condition at 200 ℃. As seen in Fig. S2, after thermal treatment, the intensity of D-band is stronger than that of G-band, indicating the amount of defective carbon is in dominant in annealed MoS2/C. The N core level spectrum of as-prepared MoS2/C and annealed MoS2/C are plotted in Fig. S1d and Fig. 4d. According to the previous report [8], the detected N core level spectrum spectrum ranging from 395.0 to 402.0 eV can be divided into three peaks. The peaks centered at about 396.9, 398.6 and 401.1 eV are attributed to the formation of N–Mo bond, C–N bond and amino, respectively [48, 49]. The results also indicate that N is doped into MoS 2 as well as carbon in both two samples. The main advantages of N-doped carbon can be concluded as follow. Firstly, N-doping can effectively enhance the electronic conductivity of carbon, which is important complement to pure MoS2 with low conductivity [44, 50]. Secondly, N-doping carbon possesses the N-groups with relative high chemical activity that is capable to binding with Li or Na ion [51, 52]. Thirdly, the doped-N can expand the interlayer distance of the carbon, which would enhance the Na adsorption capability [53, 54]. By the way, the peak located at about 395.6 eV is assigned to Mo 3p3/2 [48]. In addition, the content of carbon, nitrogen and sulfur in the as-prepared MoS2/C and annealed MoS2/C has been tested by carbon-sulfur analyzer (CS-444) and nitrogen-oxygen analyzer (TC-436), respectively. As shown in the Table S1, the content of carbon and nitrogen in the annealed MoS 2/C are 15.67 7
and 3.54 wt.%, respectively. For the as-prepared MoS2/C, the larger value of carbon and nitrogen can be attributed to some attached groups, such as -COOH, -NH2, after the process of hydrothermal sulfuration [8, 46]. The formation mechanism of the urchin-like MoS2/C nanospheres is proposed by observing the SEM images with various sulfuration time. As shown in Fig. 5a, the fresh MoMOFs presents a micron rod-like morphology with smooth surface, which agrees with previous literature [39]. After sulfuration reaction for 1.5 hour, the surface of the micro-rods was covered with disordered nanosheets (Fig. 5b). With further prolonging the reaction time for 3 hour, a large amount of irregular nanospheres emerged on the surface, which was related to crystal nucleation (Fig. 5c). Previous reports have shown that the glucose can help crystal nucleation under hydrothermal conditions easily [55, 56]. When sulfuration reaction for 12h, only aggregated nanospheres are presented (Fig. 5d). The TEM images of as-prepared MoS2/C (Fig. S3a-b) exhibited an urchin-like nanospheres morphology with hollow structure. As seen in the high-resolution TEM image (Fig. S3c-d), the existence of obvious amorphous carbon layer confirmed the carbonization during the sulfuration process. However, because of the low crystallinity, there is no clear lattice fringe. Observed from the morphology evolution with the reaction time, we can conclude that the crystal nucleation of MoS2 was formed on the Mo-MOFs precursors, and the nanospheres peeled off from the Mo-MOFs micro-rods. Finally, the sulfuration products were heated under Ar atmosphere to improve the crystallinity. Then, the nitrogen doped hollow MoS2/C nanospheres were obtained, as shown in the Fig. 2. The electrochemical properties of N doped urchin-like MoS2/C composites have been evaluated as anode materials for SIBs. The CV result of MoS2/C electrode was presented in Fig. 6a. In the first cycle, there are two obvious sodiation peaks at about 0.68 and 0.056 V, which correspond to the process of sodium intercalation (MoS2 + x Na+ +x e- → NaxMoS2) [57] and the reaction of subsequent conversion (4Na+ + MoS2 + 4e- → Mo + 2Na2S) [22], respectively. In the subsequent cycles, the reduction peak above 0.5 V disappeared, but that 8
below 0.5 V is still remained, indicating the reversible conversion reaction (4Na+ + MoS2 + 4e- → Mo + 2Na2S). As shown in the discharge/charge voltage profiles tested at 100 mA g -1 (Fig. 6b), there are visible discharge/charge plateaus in the initial cycle. From 2nd cycle, the plateaus are vanished, which is in accord with the CV curves in Fig. 6a. Besides, the MoS2/C electrode can achieve a high discharge capacity of 972 mA h g -1, with the coulombic efficiency of 61.7% for the 1st cycle (Fig. 6c). As known, the unsatisfactory initial coulombic efficiency is blamed on the formation of solid electrode interphase (SEI) [9, 22]. And then the capacity becomes stable after the first cycle. In contrast, due to the low conductivity, the capacity of pure MoS2 electrode rapidly decreases from 758 mA h g-1 at the 1st cycle to 235 mA h g-1 at the 30th cycle. As shown in Fig. 6d, the rate behavior of the MoS2 electrode is evaluated at mutative current densities (100-5000 mA g-1). The pure MoS2 exhibits much poor rate capability. The N-doped MoS2/C materials delivered a discharge capacity of 535, 466, 381, 304, 267 and 242 mA h g -1 at various current densities of 100, 200, 500, 1000, 2000 and 5000 mA g-1, respectively. When the current density is turned back to 100 mA g-1, the coin cell displayed a stable discharge capacity of 413 mA h g-1. Hindered by the high chargetransfer resistance, the capacity of pure MoS2 decreases seriously at large current density. Impressively, N-doped hollow MoS2/C nanospheres also exhibit outstanding long-term cycling performance at 2000 mA g-1, which can be attributed to the introduction of N-doped carbon (Fig. 6e). The initial specific discharge capacity achieves as high as 804 mA h g-1. After 5000 cycles, the capacity maintains around 128 mA h g-1, corresponding to a fading rate of 0.036% per cycle. As displayed in the Fig. S4, the electrode of N-doped MoS2/C maintains its hollow spheres-like morphology after long time cycling, although the nanosheets around the nanospheres were not distinct. This indicates that nitrogen doped urchin-like MoS2/C hollow structures was greatly beneficial to the reversible intercalation/de-intercalation reaction during the long-term cycling. To further analyze the long-term cycling stability of N-doped MoS2/C 9
electrode, Nyquist plots before and after long-term cycles are recorded in Fig. S5. As known, the medium frequency semicircle corresponds to the charge-transfer resistance (Rct) of the electrodes [9, 17]. The Rct of N-doped MoS2/C (499 Ω) is much smaller than that of the fresh MoS2 (820 Ω), indicating the improved conductivity by introduction of carbon. After 500 charge-discharge cycles at 2000 mA g-1, the Rct of N-doped MoS2/C is slightly increased to 606 Ω, suggesting a stable cycling stability. As listed in Table S2, the electrochemical property of the N-doped MoS2/C is compared with those of reported MoS2-based anode materials for SIBs. It is a prevalent strategy to composite MoS2 with graphene [18, 19, 58, 59]. which demonstrated good performance, but the rate capability or cycling performance need to be further improved. For example, Xiang et al. utilized the 3D graphene foams to improve the conductivity of MoS 2, but it suffered from low rate capacity of 172 mA h g -1 at 1250 mA g-1 and high capacity fading rate of 1.74% per cycle [59]. The MoS2 nanosheets displayed high capacity at low current density, but the durability is inferior [60, 61]. MoS2/carbon nanofibers composites prepared by Xiong et al. possess good capacity retention, but the discharged capacity at each current density is unsatisfactory [16]. Obviously, the nitrogen doped MoS2/C hollow structures reported in this work possess much better rate property and long-term cycling performance than most of reported MoS2-based anode materials for SIBs. As analyzed above, the excellent electrochemical performance benefits from the following three aspects. Firstly, the annealed MoS2/C presents hollow structure with high surface area. It is able to facilitate electrolyte penetration as well as accommodate the volume change during discharge-charge process, resulting in the outstanding cycling performance. Secondly, due to the decoration of uniform carbon with doped nitrogen, the annealed MoS2/C possesses satisfied electronic conductivity, which is beneficial to good rate performance. In addition, the annealed MoS 2/C was coated by carbon nanosheets that can provide enhanced structural stability.
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4. Conclusion In summary, we have demonstrated a novel strategy for the synthesis of the nitrogen doped urchin-like MoS2/C by using Mo-MOFs as precursors. This material provides good conductivity and the ability to compromise the volume change during discharge and charge. As applied to SIBs, the as-prepared MoS2/C anode exhibits amazing electrochemical performance. Typically, the electrode delivers a high initial discharge capacity of 972 mA h g 1
at 100 mA g-1. When cycled at a high current density of 2000 mA g -1, it can achieve a low
capacity fading rate (0.036% per cycle) for 5000 cycles. As a result, the N-doped urchin-like MoS2/C anode may be a promising candidate for long-life sodium-ion batteries.
Acknowledgements This work was supported by National High Technology Research and Development Program of China (863 Program) (Grant no. 2013AA110106), National Natural Science Foundation of China (Grant no. 51374255 and 51572299) and the Fundamental Research Funds for the Central Universities of Central South University (Grant no. 2017zzts004).
References [1] W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Selfsacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for high-rate sodium-ion full batteries, Nano Energy 25 (2016) 145-153. [2] Y. Xu, Q. Wei, C. Xu, Q. Li, Q. An, P. Zhang, J. Sheng, L. Zhou, L. Mai, Layer-by-layer Na3V2(PO4)3 embedded in reduced graphene oxide as superior rate and ultralong-life sodiumion battery cathode, Adv. Energy Mater. 6 (2016) 1600389. [3] S. Yuan, Y.B. Liu, D. Xu, D.L. Ma, S. Wang, X.H. Yang, Z.Y. Cao, X.B. Zhang, Pure single-crystalline Na1.1V3O7.9 nanobelts as superior cathode materials for rechargeable sodium-ion batteries, Adv. Sci. 2 (2015) 1400018. [4] F. Xiong, S. Tan, Q. Wei, G. Zhang, J. Sheng, Q. An, L. Mai, Three-dimensional graphene frameworks wrapped Li3V2(PO4)3 with reversible topotactic sodium-ion storage, Nano Energy 11
32 (2017) 347-352. [5] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Na-ion batteries, recent advances and present challenges to become low cost energy storage systems, Energy Environ. Sci. 5 (2012) 5884-5901. [6] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries, Adv. Energy Mater. 2 (2012) 710-721. [7] Y.F. Dong, S. Li, K.N. Zhao, C.H. Han, W. Chen, B.L. Wang, L. Wang, B.A. Xu, Q.L. Wei, L. Zhang, X. Xu, L.Q. Mai, Hierarchical zigzag Na1.25V3O8 nanowires with topotactically encoded superior performance for sodium-ion battery cathodes, Energy Environ. Sci. 8 (2015) 1267-1275. [8] Z.T. Shi, W. Kang, J. Xu, Y.W. Sun, M. Jiang, T.W. Ng, H.T. Xue, D.Y.W. Yu, W. Zhang, C.-S. Lee, Hierarchical nanotubes assembled from MoS 2-carbon monolayer sandwiched superstructure nanosheets for high-performance sodium ion batteries, Nano Energy 22 (2016) 27-37. [9] Q.Z. Yan, Y. Lu , N. Zhang , K.X. Lei , F.J. Li , J. Chen, Facile spraying synthesis and high-performance sodium storage of mesoporous MoS2/C microspheres, Adv. Funct. Mater. 26 (2016) 911-918. [10] X. Hu, W. Zhang, X. Liu, Y. Mei, Y. Huang, Nanostructured Mo-based electrode materials for electrochemical energy storage, Chem. Soc. Rev. 44 (2015) 2376-2404. [11] J. Zhou, G. Fang, A. Pan, S. Liang, Oxygen-incorporated MoS2 nanosheets with expanded interlayers for hydrogen evolution reaction and pseudocapacitor applications, ACS Appl. Mater. Inter. 8 (2016) 33681-33689. [12] M. Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, The chemistry of twodimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 5 (2013) 263275. [13] H. Jiang, D. Ren, H. Wang, Y. Hu, S. Guo, H. Yuan, P. Hu, L. Zhang, C. Li, 2D monolayer MoS2-carbon interoverlapped superstructure: engineering ideal atomic interface for lithium ion storage, Adv. Mater. 27 (2015) 3687-3695. [14] J. Wang, C. Luo, T. Gao, A. Langrock, A.C. Mignerey, C. Wang, An advanced MoS2/carbon anode for high-performance sodium-ion batteries, Small 11 (2015) 473-481. [15] X. Xiong, W. Luo, X. Hu, C. Chen, L. Qie, D. Hou, Y. Huang, Flexible membranes of MoS2/C nanofibers by electrospinning as binder-free anodes for high-performance sodium-ion batteries, Sci. Rep. 5 (2015) 9254. 12
[16] C. Zhu, X. Mu, P.A. van Aken, Y. Yu, J. Maier, Single-layered ultrasmall nanoplates of MoS2 embedded in carbon nanofibers with excellent electrochemical performance for lithium and sodium storage, Angew. Chem. Int. Ed. 53 (2014) 2152-2156. [17] S.H. Choi, Y.N. Ko, J.K. Lee, Y.C. Kang, 3D MoS 2-graphene microspheres consisting of multiple nanospheres with superior sodium ion storage properties, Adv. Funct. Mater. 25 (2015) 1780-1788. [18] L. David, R. Bhandavat, G. Singh, MoS 2/graphene composite paper for sodium-ion battery electrodes, ACS Nano 8 (2014) 1759-1770. [19] X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, MoS2/graphene composite anodes with enhanced performance for sodium-ion batteries: the role of the two-dimensional heterointerface, Adv. Funct. Mater. 25 (2015) 1393-1403. [20] L. Oakes, R. Carter, T. Hanken, A.P. Cohn, K. Share, B. Schmidt, C.L. Pint, Interface strain in vertically stacked two-dimensional heterostructured carbon-MoS2 nanosheets controls electrochemical reactivity, Nat. Commun. 7 (2016) 11796. [21] K. Pramoda, U. Gupta, I. Ahmad, R. Kumar, C.N.R. Rao, Assemblies of covalently cross-linked nanosheets of MoS2 and of MoS2–RGO: synthesis and novel properties, J. Mater. Chem. A 4 (2016) 8989-8994. [22] X. Xu, D. Yu, H. Zhou, L. Zhang, C. Xiao, C. Guo, S. Guo, S. Ding, MoS 2 nanosheets grown on amorphous carbon nanotubes for enhanced sodium storage, J. Mater. Chem. A 4 (2016) 4375-4379. [23] H. Deng, S. Grunder, K.E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gándara, A.C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki, J.F. Stoddart, O.M. Yaghi, Large-pore apertures in a series of metal-organic frameworks, Science 336 (2012) 1018-1023. [24] H. Hayashi, A.P. Cote, H. Furukawa, M. O/'Keeffe, O.M. Yaghi, Zeolite A imidazolate frameworks, Nat. Mater. 6 (2007) 501-506. [25] W. Lu, Z. Wei, Z.Y. Gu, T.F. Liu, J. Park, J. Park, J. Tian, M. Zhang, Q. Zhang, T. Gentle Iii, M. Bosch, H.-C. Zhou, Tuning the structure and function of metal-organic frameworks via linker design, Chem. Soc. Rev. 43 (2014) 5561-5593. [26] N. Stock, S. Biswas, Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites, Chem. Rev. 112 (2012) 933-969. [27] Y. Zhao, Z. Song, X. Li, Q. Sun, N. Cheng, S. Lawes, X. Sun, Metal organic frameworks for energy storage and conversion, Energy Storage Mater. 2 (2016) 35-62. [28] D. Ji, H. Zhou, Y. Tong, J. Wang, M. Zhu, T. Chen, A. Yuan, Facile fabrication of MOF13
derived octahedral CuO wrapped 3D graphene network as binder-free anode for high performance lithium-ion batteries, Chem. Eng. J. 313 (2017) 1623-1632. [29] B. Liu, H. Shioyama, T. Akita, Q. Xu, Metal-organic framework as a template for porous carbon synthesis, J. Am. Chem. Soc. 130 (2008) 5390-5391. [30] F. Zheng, Y. Yang, Q. Chen, High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework, Nat. Commun. 5 (2014) 5261. [31] Y. Zhang, A. Pan, Y. Wang, W. Wei, Y. Su, J. Hu, G. Cao, S. Liang, Dodecahedron-shaped porous vanadium oxide and carbon composite for high-rate lithium ion batteries, ACS Appl. Mater. Inter. 8 (2016) 17303-17311. [32] A.J. Amali, J.K. Sun, Q. Xu, From assembled metal-organic framework nanoparticles to hierarchically porous carbon for electrochemical energy storage, Chem. Commun. 50 (2014) 1519-1522. [33] H. Wang, Q.L. Zhu, R. Zou, Q. Xu, Metal-organic frameworks for energy applications, Chem 2 (2017) 52-80. [34] Q.L. Zhu, W. Xia, T. Akita, R. Zou, Q. Xu, Metal-organic framework-derived honeycomb-like open porous nanostructures as precious-metal-free catalysts for highly efficient oxygen electroreduction, Adv. Mater. 28 (2016) 6391-6398. [35] G. Fang, J. Zhou, C. Liang, A. Pan, C. Zhang, Y. Tang, X. Tan, J. Liu, S. Liang, MOFs nanosheets derived porous metal oxide-coated three-dimensional substrates for lithium-ion battery applications, Nano Energy 26 (2016) 57-65. [36] J. Huang, G. Fang, K. Liu, J. Zhou, X. Tang, K. Cai, S. Liang, Controllable synthesis of highly uniform cuboid-shape MOFs and their derivatives for lithium-ion battery and photocatalysis applications, Chem. Eng. J. 322 (2017) 281-292. [37] G.Z. Fang, J. Zhou, Y.S. Cai, S.N. Liu, X.P. Tan, A.Q. Pan, S.Q. Liang, Metal-organic frameworks templated two dimensional hybrid bimetallic metal oxides with enhanced lithium/sodium storage capability, J. Mater. Chem. A, DOI: 10.1039/C7TA01961K. [38] P. Martin-Zarza, J.M. Arrieta, M. Carmen MuAoz-RocaC, P. Gili, Synthesis and characterization of new octamolybdates containing imidazole, 1-Methyl- or 2-Methylimidazole Co-ordinatively bound to molybdenum, J. Chem. Soc. Dalton Trans. (1993) 15511557. [39] X. Cao, B. Zheng, W. Shi, J. Yang, Z. Fan, Z. Luo, X. Rui, B. Chen, Q. Yan, H. Zhang, Reduced graphene oxide-wrapped MoO3 composites prepared by using metal-organic frameworks as precursor for all-solid-state flexible supercapacitors, Adv. Mater. 27 (2015) 4695-4701. 14
[40] Z. Hu, L. Wang, K. Zhang, J. Wang, F. Cheng, Z. Tao, J. Chen, MoS 2 nanoflowers with expanded interlayers as high-performance anodes for sodium-ion batteries, Angew. Chem. Int. Ed. 126 (2014) 13008-13012. [41] A. Zak, Y. Feldman, V. Lyakhovitskaya, G. Leitus, R. Popovitz-Biro, E. Wachtel, H. Cohen, S. Reich, R. Tenne, Alkali metal intercalated fullerene-like MS2 (M = W, Mo) nanoparticles and their properties, J. Am. Chem. Soc. 124 (2002) 4747-4758. [42] D.W. Su, S.X. Dou, G.X. Wang, Hierarchical orthorhombic V2O5 hollow nanospheres as high performance cathode materials for sodium-ion batteries, J. Mater. Chem. A 2 (2014) 11185-11194. [43] X. Fan, P. Xu, D. Zhou, Y. Sun, Y.C. Li, M.A.T. Nguyen, M. Terrones, T.E. Mallouk, Fast and efficient preparation of exfoliated 2H MoS2 nanosheets by sonication-assisted lithium intercalation and infrared laser-induced 1T to 2H phase reversion, Nano Lett. 15 (2015) 59565960. [44] J. Shao, T. Gao, Q. Qu, Q. Shi, Z. Zuo, H. Zheng, Ultrafast Li-storage of MoS2 nanosheets grown on metal-organic framework-derived microporous nitrogen-doped carbon dodecahedrons, J. Power Sources 324 (2016) 1-7. [45] M.A. Bissett, I.A. Kinloch, R.A.W. Dryfe, Characterization of MoS 2–graphene composites for high-performance coin cell supercapacitors, ACS Appl. Mater. Inter. 7 (2015) 17388-17398. [46] M. Sevilla, A.B. Fuertes, The production of carbon materials by hydrothermal carbonization of cellulose, Carbon 47 (2009) 2281-2289. [47] L. Lai, G. Huang, X. Wang, J. Weng, Solvothermal syntheses of hollow carbon microspheres modified with –NH2 and –OH groups in one-step process, Carbon 48 (2010) 3145-3156. [48] S. Dong, X. Chen, K. Zhang, L. Gu, L. Zhang, X. Zhou, L. Li, Z. Liu, P. Han, H. Xu, J. Yao, C. Zhang, X. Zhang, C. Shang, G. Cui, L. Chen, Molybdenum nitride based hybrid cathode for rechargeable lithium-O2 batteries, Chem. Commun. 47 (2011) 11291-11293. [49] Y.H. Lai, C.T. Yeh, J.M. Hwang, H.L. Hwang, C.T. Chen, W.H. Hung, Sputtering and Eetching of GaN surfaces, The J. Phys. Chem. B 105 (2001) 10029-10036.. [50] J.M. Jeong, K.G. Lee, S.J. Chang, J.W. Kim, Y.K. Han, S.J. Lee, B.G. Choi, Ultrathin sandwich-like MoS2@N-doped carbon nanosheets for anodes of lithium ion batteries, Nanoscale 7 (2015) 324-329. [51] Y. Qiu, W. Li, W. Zhao, G. Li, Y. Hou, M. Liu, L. Zhou, F. Ye, H. Li, Z. Wei, S. Yang, W. Duan, Y. Ye, J. Guo, Y. Zhang, High-rate, ultralong cycle-life lithium/sulfur batteries enabled 15
by nitrogen-doped graphene, Nano Lett. 14 (2014) 4821-4827. [52] S. Xiao, S. Liu, J. Zhang, Y. Wang, Polyurethane-derived N-doped porous carbon with interconnected sheet-like structure as polysulfide reservoir for lithium–sulfur batteries, J. Power Sources 293 (2015) 119-126. [53] L. Fu, K. Tang, K. Song, P.A. van Aken, Y. Yu, J. Maier, Nitrogen doped porous carbon fibres as anode materials for sodium ion batteries with excellent rate performance, Nanoscale 6 (2014) 1384-1389. [54] Z. Wang, L. Qie, L. Yuan, W. Zhang, X. Hu, Y. Huang, Functionalized N-doped interconnected carbon nanofibers as an anode material for sodium-ion storage with excellent performance, Carbon 55 (2013) 328-334. [55] Y. Xia, B. Wang, X. Zhao, G. Wang, H. Wang, Core-shell composite of hierarchical MoS2 nanosheets supported on graphitized hollow carbon microspheres for high performance lithium-ion batteries, Electrochim. Acta 187 (2016) 55-64. [56] D. Li, Y. Gong, Y. Zhang, C. Luo, W. Li, Q. Fu, C. Pan, Facile synthesis of carbon nanosphere/NiCo2O4 core-shell sub-microspheres for high performance supercapacitor, Sci. Rep-UK 5 (2015) 12903. [57] X. Wang, X. Shen, Z. Wang, R. Yu, L. Chen, Atomic-scale clarification of structural transition of MoS2 upon sodium intercalation, ACS Nano 8 (2014) 11394-11400. [58] Y.X. Wang, S.L. Chou, D. Wexler, H.K. Liu, S.X. Dou, High-performance sodium-ion batteries and sodium-ion pseudocapacitors based on MoS2/graphene composites, Chem-Eur J. 20 (2014) 9607-9612. [59] J. Xiang, D. Dong, F. Wen, J. Zhao, X. Zhang, L. Wang, Z. Liu, Microwave synthesized self-standing electrode of MoS2 nanosheets assembled on graphene foam for highperformance Li-ion and Na-ion batteries, J. Alloy. Compd. 660 (2016) 11-16. [60] S. Zhang, X. Yu, H. Yu, Y. Chen, P. Gao, C. Li, C. Zhu, Growth of ultrathin MoS 2 nanosheets with expanded spacing of (002) plane on carbon nanotubes for high-performance sodium-ion battery anodes, ACS Appl. Mater. Inter. 6 (2014) 21880-21885. [61] D. Su, S. Dou, G. Wang, Ultrathin MoS2 nanosheets as anode materials for sodium-ion batteries with superior performance, Adv. Energy Mater. 5 (2015) 1401205.
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Figures and captions
Scheme 1 Illustration of preparation process of N doped urchin-like MoS2/C.
Fig. 1. XRD pattern of (a) Mo-MOFs, (b) as-prepared MoS2/C and annealed MoS2/C.
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Fig. 2 (a-b) TEM images and (c-d) HRTEM image of the N doped urchin-like MoS2/C.
18
Fig. 3. (a) N2 adsorption/desorption isotherm and (b) corresponding BJH pore-size distribution curve of annealed MoS2/C.
19
Fig. 4 Core level XPS spectrum of annealed MoS2/C.
20
Fig. 5 FESEM images at various sulfuration time: (a) fresh Mo-MOFs, (b) 1.5 hour, (c) 3 hour and (d) 12 hour.
21
Fig. 6 Electrochemical performance of N-doped MoS2/C in SIBs, in comparison with the pure MoS2: (a) the first three CV curves at a scan rate of 0.1 mV s -1, (b) the charge/discharge profiles at 100 mA g-1, (c) the cycling performance and corresponding coulombic efficiency at 100 mA g-1, (d) the rate performance at various current density from 100 to 5000 mA g -1, (e) the high-rate (2000 mA g-1) cycling performance and corresponding coulombic efficiency.
22
Highlights N-doped MoS2/C is successfully synthesized by using Mo-MOFs as the precursors. The formation mechanism of the hollow MoS2/C nanospheres is proposed. The MoS2 anode demonstrates superior long-term cycling performance for SIBs.
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S1385-8947(17)31097-5 http://dx.doi.org/10.1016/j.cej.2017.06.146 CEJ 17227
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
12 April 2017 9 June 2017 25 June 2017
Please cite this article as: Y. Cai, H. Yang, J. Zhou, Z. Luo, G. Fang, S. Liu, A. Pan, S. Liang, Nitrogen Doped Hollow MoS2/C Nanospheres as Anode for Long-life Sodium-Ion Batteries, Chemical Engineering Journal (2017), doi: http://dx.doi.org/10.1016/j.cej.2017.06.146
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen Doped Hollow MoS2/C Nanospheres as Anode for Long-life Sodium-Ion Batteries Yangsheng Caia,b, Hulin Yanga,b, Jiang Zhoua,b,*, Zhigao Luoa,b, Guozhao Fanga,b, Sainan Liua,b, Anqiang Pana,b,*, and Shuquan Lianga,b,* a
School of Materials Science and Engineering, Central South University, Changsha 410083, P.
R. China b
Key Laboratory of Nonferrous Metal Materials Science and Engineering, Ministry of
Education, Central South University, Changsha 410083, P. R. China
Abstract As a complement or alternative to lithium ion batteries, development of highperformance sodium ion batteries with long cycle life is urgent. Here, nitrogen doped urchinlike MoS2/C hollow structures are successfully synthesized by using Mo-MOFs as the precursors with a facile sulfuration process. The nanocomposites that contain carbon and nitrogen elements possess good electronic conductivity as well as expanded interlayer, which are beneficial for electron transportation and ion diffusion. The hierarchical hollow structure is able to accommodate volume change during discharge/charge. When applied as anode for sodium-ion batteries, the N-doped MoS2/C anode exhibits good rate capability and outstanding cycling performance. It delivers a high initial discharge capacity of 972 mA h g -1 at 100 mA g−1. The discharge capacity can achieve 242 mA h g-1 even at 5000 mA g-1. Importantly, long-term cycling test shows that the capacity maintains a considerable capacity of 128 mA h g-1 after 5000 cycles at 2 A g-1, with a capacity fading rate of 0.036% per cycle. Keywords: molybdenum disulfide, metal organic frameworks, hollow structure, nitrogen doped, sodium-ion batteries
Corresponding author: Tel.: +86 0731-88836069. Fax: +86 0731-88876692. E-mail address: [email protected] (J. Zhou), [email protected] (A. Pan), [email protected] (S. Liang)
1
1. Introduction Sodium-ion batteries (SIBs) are gradually regarded as a promising candidate for largescale energy storage system due to the abundant Na element in nature and almost the same electrochemical mechanism to lithium-ion batteries (LIBs) [1-4]. Actually, due to the inherent large ionic radius (RNa=1.02 Å) and heavy relative atomic mass (uNa=22.9898), SIBs usually exhibit unsatisfactory electrochemical performance, especially the low specific capacity and poor cycling stability [5-7]. Therefore, it is considerable to develop desirable electrode materials for SIBs. Molybdenum disulfide (MoS2) with a large interlayer spacing (d=0.62 nm), exhibiting a reversible capacity as high as 670 mA h g−1, may be an attractive anode materials for sodium storage [8-11]. Particularly, the 2D layered structure in MoS2 is constructed by the stack of three atomic layers (S−Mo−S) through Van der Waals interactions, which is beneficial to the intercalation/de-intercalation of Na+ ions [12]. However, the practical application of bare MoS2 as SIBs is suffered from the low electronic conductivity and great volume variation [8, 9, 13]. It is a validated strategy to overcome above problem by decorating MoS2 with carbon-based materials [8, 9, 14-22]. For example, the novel MoS2/carbon-nanotubes delivered a remarkable reversible capacity (461 mA h g-1 at 500 mA g-1) as anode material for sodium-ion cell [22]. Shi et al. assembled MoS2/C hierarchical nanotubes, exhibiting remarkable rate behavior as SIBs anodes [8]. The MoS2/C microspheres also manifest an amazing long-life performance of 390 mA h g−1 at 1000 mA g−1 after 2500 cycles [9]. Nevertheless, there is still an opportunity to enhance MoS2 based anode materials with excellent cycling stability at high-rate capability. The architecture of metal ions and organic linkers, metal organic frameworks (MOFs), have become increasingly fashionable on account of novel structure, chemical stability and promising carbonaceous ramification [23-28]. The pioneering work on the use of MOFs as template/precursor for the synthesis of porous carbon has been reported by Xu et al. in 2008 [29]. Zheng et al. fabricated an N-doped carbon materials derived from ZIF-8, which 2
displayed the enhanced ability of ion diffusion and electron transportation for energy storage [30]. Specially, the self-templated carbon and nitrogen source derived from MOFs are considered as desirable conductive complements to improve the electrochemical performance of transition metal-based compounds [29-34]. Zhu et al. synthesised a Co9S8/carbon composite using MOFs as precursor, which exhibited outstanding electrocatalytic performance [34]. It is a popular application of the electrode materials for energy storage applications, especially the metal oxides and sulfides (selenides), which transformed from MOFs [33-37]. Our group has derived various MOFs to porous metal oxide [35], ternary oxide [36], hybrid bimetallic metal oxides [37] and so on, which all exhibit excellent electrochemical performance. To the best of our knowledge, there is no report concerning about the self-templated synthesis of MoS2 nanostructures by the use of Mo-MOFs. In this work, we report the synthesis of nitrogen doped MoS2/C hollow structures with expanded interlayers by sulfuration of Mo-MOFs. The formation mechanism of the urchinlike morphology is investigated. Benefit from the composited carbon and nitrogen, the present MoS2 is supposed to possess high conductivity. As a proof, the MoS2/C nanocomposites are employed as anode materials for SIBs, which exhibit good rate capability and excellent longterm cycling performance.
2. Experimental section 2.1 Materials Synthesis. Molybdenum-based MOFs was synthesized according to the previous literature [38]. Typically, 3.5 g MoO3 and 1.66 g imidazole were mixed in a 500 mL round-bottom flask. Then 250 mL deionized (DI) water was added under vigorous stirring overnight by a reflux device. The white powders, Mo-MOFs, were collected by suction filtration with DI water and dried in vacuum at 50 °C for 12 hours.
3
To obtain hollow N-doped MoS2/C nanospheres, the Mo-MOFs were treated via a sulfuration process. Briefly, 100 mg Mo-MOFs, 100 mg glucose and 400 mg thiocarbamide were dispersed in 20 mL DI water by sonication for 15 min. Then, the suspension was poured into a 50 mL Teflon-lined stainless-steel autoclave and reacted at 200 ℃ for 12 h. The black precipitate (as-prepared MoS2/C) was collected by suction filtration with DI water and ethanol, and dried in a vacuum overnight at 50 ℃. To obtain the final product (annealed MoS2/C), the black precipitate was heated under Ar atmosphere at 700 ℃ for 2 hours with a heating rate of 5 min-1. For comparison, pure MoS2 were prepared by using a similar method, except that the Mo-MOFs was replaced by the Na2MoO4, and the glucose was not added to the reaction. 2.2 Material characterization The as-synthesized Mo-MOFs and MoS2/C were characterized using X-ray diffractometer (XRD, Rigaku D/max 2500, Cu), field-emission scanning electron microscopeenergy dispersive X-ray analysis (FESEM-EDS, FEI Nova Nano-SEM 230), transmission electron microscopy (TEM, JEOL-JEM-2100F). The valence state of the element in MoS2/C was determined by an X-ray photoelectron spectra analyses (XPS, ESCALAB 250Xi). The BET surface area and pore distribution of MoS 2/C were measured by a Quantachrome Instruments (NOVA 4200e). The content of carbon, nitrogen and sulfur in the samples have been tested by carbon-sulfur analyzer (CS-444) and nitrogen-oxygen analyzer (TC-436), respectively. 2.3. Electrochemical measurements The electrochemical properties of were conducted in CR 2016 coin cells assembled in a professional glove box (Mbraun, Garching, Germany). The cathode electrodes were prepared by dispersing MoS2/C (70 wt.%), acetylene black (20 wt.%) and polyvinylidene fluoride binder (10 wt.%) in N-methyl-2-pyrrolidone solution as slurry, which were next coated on copper foils and dried in a vacuum oven at 100℃ for 12 h. The sodium metal plates and glass 4
fiber were performed as counter electrodes and separator. A commercial solution containing 1 M NaClO4 and ethylene carbonate/dimethyl carbonate (EC/DMC; 1:1, v/v) was used as electrolyte.
The
measurements
of
cyclic
voltammetry
(CV)
and
galvanostatic
charge/discharge were finished by utilizing the CHI604E electrochemical workstation (Chenhua, Shanghai, China) and Land battery tester (Land CT 2001A, China) with the testing voltage range from 0.01 to 3 V versus Na+/Na.
3. Results and Discussion The urchin-like MoS2 nanocomposites were synthesized by a facile sulfuration approach used Mo-MOF as precursors (Scheme 1). As shown in Fig. 1a, the XRD peaks of assynthesized Mo-MOFs is in accord well with previous investigation [39], confirming the successful preparation of Mo-MOFs. After the treatment of sulfuration, the X-ray diffraction pattern (shown in Fig. 1b) of the as-obtained black powders can be indexed to hexagonal 2HMoS2 phase (space group: P63/mmc (194), a = 3.16116 Å, b = 3.16116 Å, c = 12.2985 Å, JCPDS card No. 37-1492). As shown in the XRD pattern of as-prepared MoS2/C (after hydrothermal reaction), there are two peaks at 2θ=33.2 and 57.1° that can be indexed to (100) and (110) planes of hexagonal 2H-MoS2. After post-annealing at 700 ℃, the (002) diffraction peak at 2θ=13.44° appears, and the intensity of other peaks is also reinforced, demonstrating the increased crystallinity of annealed MoS2/C. Notably, the (002) peak was shift toward low angles compared with the standard doubling angles (14.38°), indicating the expanded interlayer distance from 6.15 to 6.49 Å. The expanded interlayer may be due to the incorporation of foreign atoms or molecules, such as oxygen atom or NH 4+ [11, 40, 41], which would be advantageous for Na+ ion diffusion. The morphology of the products was observed by TEM. As shown in Fig. 2a, the annealed MoS2/C presents a uniform morphology of urchin-like nanospheres with hollow interior. The magnified TEM image shown in Fig. 2b further confirmed the hierarchical 5
structures, whose external and inner diameter are about 400 and 200 nm, respectively. According to the previous reports, the hollow nanospheres structure is beneficial for battery application, as this structure can accommodate the volume change during discharge-charge process [17, 42]. From an enlarged observation of the edge (Fig. 2c), there are numberless stacked 2D MoS2 nanosheets and amorphous carbon layer is coated the nanosheets. The lattice fringes with spacing of 0.656, 0.287 and 0.234 nm in the high-resolution TEM image (Fig. 2c-d) are clearly marked, which can be indexed to (002), (100) and (103) crystal faces, respectively. The nitrogen adsorption/desorption analysis was executed to acquire the surface information of the annealed MoS2/C. A type-IV isotherm (Fig. 3a) indicates that the annealed MoS2/C possesses mesoporous structure. The BET specific surface area of the annealed MoS2/C is as high as 51.04 m2 g-1, which could be beneficial for SIBs applications. The BJH curve (Fig. 3b) illustrates a sharp peak at 3.8 nm, confirming the presence of mesoporous channels in the urchin-like hollow MoS2/C architectures. The XPS analysis was employed to investigate the chemical state of the as-prepared MoS2/C and annealed MoS2/C nanocomposites. Fig. S1a (in Electronic Supporting Information (ESI†)) depicts the Mo core level XPS spectra of the as-prepared MoS2/C. There are two pairs of peaks (the one are at 231.8 and 228.5 eV, the other are at 233.2 and 229.6 eV), corresponding to 1T-MoS2 and 2H-MoS2, respectively [43]. A small peak at 225.7 eV is related to S 2s [8, 14]. As shown in the Mo core level XPS spectra (Fig. 4a), there is only a pair of distinct peaks at 232.9 eV for Mo 3d3/2 and 229.7 eV for Mo 3d5/2, identifying with the 2H-MoS2 phase [8, 44]. The disappearance of characteristic peak for 1T-MoS2 indicated that the metastable phase was transformed to 2H-MoS2 after thermal treatment. The peak at low binding energy (236.0 eV) is attributed to oxidation state of Mo (Mo6+), which results from the surface oxidation of MoS2 [44, 45]. In addition, the S 2p core level spectra of both asprepared MoS2/C and urchin-like MoS2/C (Fig. S1b and Fig. 4b) present a couple of peaks, 6
corresponding to S 2p1/2 and S 2p3/2, demonstrating the presence of S2- ions [14, 15]. As shown in Fig. S1c and Fig. 4c, the similar C 1s spectra of as-prepared MoS2/C and annealed MoS2/C can be spilt up into three peaks at 284.6 (284.8), 285.9 (286.0) and 287.8 (287.0) eV, which can be ascribed to C–C, C–N and C–O bond [46, 47]. Furthermore, the Raman spectra (Fig. S2) also verified the existence of carbon species in the as-prepared MoS2/C and annealed MoS2/C. Both of them present a pair of prominent peaks at approximately 1345 and 1592 cm1
, which are associated with disordered carbon (D-band) and graphitic carbon (G-band),
respectively. The results indicate that carbonization of the glucose and the Mo-MOFs have occurred during the sulfuration process under hydrothermal condition at 200 ℃. As seen in Fig. S2, after thermal treatment, the intensity of D-band is stronger than that of G-band, indicating the amount of defective carbon is in dominant in annealed MoS2/C. The N core level spectrum of as-prepared MoS2/C and annealed MoS2/C are plotted in Fig. S1d and Fig. 4d. According to the previous report [8], the detected N core level spectrum spectrum ranging from 395.0 to 402.0 eV can be divided into three peaks. The peaks centered at about 396.9, 398.6 and 401.1 eV are attributed to the formation of N–Mo bond, C–N bond and amino, respectively [48, 49]. The results also indicate that N is doped into MoS 2 as well as carbon in both two samples. The main advantages of N-doped carbon can be concluded as follow. Firstly, N-doping can effectively enhance the electronic conductivity of carbon, which is important complement to pure MoS2 with low conductivity [44, 50]. Secondly, N-doping carbon possesses the N-groups with relative high chemical activity that is capable to binding with Li or Na ion [51, 52]. Thirdly, the doped-N can expand the interlayer distance of the carbon, which would enhance the Na adsorption capability [53, 54]. By the way, the peak located at about 395.6 eV is assigned to Mo 3p3/2 [48]. In addition, the content of carbon, nitrogen and sulfur in the as-prepared MoS2/C and annealed MoS2/C has been tested by carbon-sulfur analyzer (CS-444) and nitrogen-oxygen analyzer (TC-436), respectively. As shown in the Table S1, the content of carbon and nitrogen in the annealed MoS 2/C are 15.67 7
and 3.54 wt.%, respectively. For the as-prepared MoS2/C, the larger value of carbon and nitrogen can be attributed to some attached groups, such as -COOH, -NH2, after the process of hydrothermal sulfuration [8, 46]. The formation mechanism of the urchin-like MoS2/C nanospheres is proposed by observing the SEM images with various sulfuration time. As shown in Fig. 5a, the fresh MoMOFs presents a micron rod-like morphology with smooth surface, which agrees with previous literature [39]. After sulfuration reaction for 1.5 hour, the surface of the micro-rods was covered with disordered nanosheets (Fig. 5b). With further prolonging the reaction time for 3 hour, a large amount of irregular nanospheres emerged on the surface, which was related to crystal nucleation (Fig. 5c). Previous reports have shown that the glucose can help crystal nucleation under hydrothermal conditions easily [55, 56]. When sulfuration reaction for 12h, only aggregated nanospheres are presented (Fig. 5d). The TEM images of as-prepared MoS2/C (Fig. S3a-b) exhibited an urchin-like nanospheres morphology with hollow structure. As seen in the high-resolution TEM image (Fig. S3c-d), the existence of obvious amorphous carbon layer confirmed the carbonization during the sulfuration process. However, because of the low crystallinity, there is no clear lattice fringe. Observed from the morphology evolution with the reaction time, we can conclude that the crystal nucleation of MoS2 was formed on the Mo-MOFs precursors, and the nanospheres peeled off from the Mo-MOFs micro-rods. Finally, the sulfuration products were heated under Ar atmosphere to improve the crystallinity. Then, the nitrogen doped hollow MoS2/C nanospheres were obtained, as shown in the Fig. 2. The electrochemical properties of N doped urchin-like MoS2/C composites have been evaluated as anode materials for SIBs. The CV result of MoS2/C electrode was presented in Fig. 6a. In the first cycle, there are two obvious sodiation peaks at about 0.68 and 0.056 V, which correspond to the process of sodium intercalation (MoS2 + x Na+ +x e- → NaxMoS2) [57] and the reaction of subsequent conversion (4Na+ + MoS2 + 4e- → Mo + 2Na2S) [22], respectively. In the subsequent cycles, the reduction peak above 0.5 V disappeared, but that 8
below 0.5 V is still remained, indicating the reversible conversion reaction (4Na+ + MoS2 + 4e- → Mo + 2Na2S). As shown in the discharge/charge voltage profiles tested at 100 mA g -1 (Fig. 6b), there are visible discharge/charge plateaus in the initial cycle. From 2nd cycle, the plateaus are vanished, which is in accord with the CV curves in Fig. 6a. Besides, the MoS2/C electrode can achieve a high discharge capacity of 972 mA h g -1, with the coulombic efficiency of 61.7% for the 1st cycle (Fig. 6c). As known, the unsatisfactory initial coulombic efficiency is blamed on the formation of solid electrode interphase (SEI) [9, 22]. And then the capacity becomes stable after the first cycle. In contrast, due to the low conductivity, the capacity of pure MoS2 electrode rapidly decreases from 758 mA h g-1 at the 1st cycle to 235 mA h g-1 at the 30th cycle. As shown in Fig. 6d, the rate behavior of the MoS2 electrode is evaluated at mutative current densities (100-5000 mA g-1). The pure MoS2 exhibits much poor rate capability. The N-doped MoS2/C materials delivered a discharge capacity of 535, 466, 381, 304, 267 and 242 mA h g -1 at various current densities of 100, 200, 500, 1000, 2000 and 5000 mA g-1, respectively. When the current density is turned back to 100 mA g-1, the coin cell displayed a stable discharge capacity of 413 mA h g-1. Hindered by the high chargetransfer resistance, the capacity of pure MoS2 decreases seriously at large current density. Impressively, N-doped hollow MoS2/C nanospheres also exhibit outstanding long-term cycling performance at 2000 mA g-1, which can be attributed to the introduction of N-doped carbon (Fig. 6e). The initial specific discharge capacity achieves as high as 804 mA h g-1. After 5000 cycles, the capacity maintains around 128 mA h g-1, corresponding to a fading rate of 0.036% per cycle. As displayed in the Fig. S4, the electrode of N-doped MoS2/C maintains its hollow spheres-like morphology after long time cycling, although the nanosheets around the nanospheres were not distinct. This indicates that nitrogen doped urchin-like MoS2/C hollow structures was greatly beneficial to the reversible intercalation/de-intercalation reaction during the long-term cycling. To further analyze the long-term cycling stability of N-doped MoS2/C 9
electrode, Nyquist plots before and after long-term cycles are recorded in Fig. S5. As known, the medium frequency semicircle corresponds to the charge-transfer resistance (Rct) of the electrodes [9, 17]. The Rct of N-doped MoS2/C (499 Ω) is much smaller than that of the fresh MoS2 (820 Ω), indicating the improved conductivity by introduction of carbon. After 500 charge-discharge cycles at 2000 mA g-1, the Rct of N-doped MoS2/C is slightly increased to 606 Ω, suggesting a stable cycling stability. As listed in Table S2, the electrochemical property of the N-doped MoS2/C is compared with those of reported MoS2-based anode materials for SIBs. It is a prevalent strategy to composite MoS2 with graphene [18, 19, 58, 59]. which demonstrated good performance, but the rate capability or cycling performance need to be further improved. For example, Xiang et al. utilized the 3D graphene foams to improve the conductivity of MoS 2, but it suffered from low rate capacity of 172 mA h g -1 at 1250 mA g-1 and high capacity fading rate of 1.74% per cycle [59]. The MoS2 nanosheets displayed high capacity at low current density, but the durability is inferior [60, 61]. MoS2/carbon nanofibers composites prepared by Xiong et al. possess good capacity retention, but the discharged capacity at each current density is unsatisfactory [16]. Obviously, the nitrogen doped MoS2/C hollow structures reported in this work possess much better rate property and long-term cycling performance than most of reported MoS2-based anode materials for SIBs. As analyzed above, the excellent electrochemical performance benefits from the following three aspects. Firstly, the annealed MoS2/C presents hollow structure with high surface area. It is able to facilitate electrolyte penetration as well as accommodate the volume change during discharge-charge process, resulting in the outstanding cycling performance. Secondly, due to the decoration of uniform carbon with doped nitrogen, the annealed MoS2/C possesses satisfied electronic conductivity, which is beneficial to good rate performance. In addition, the annealed MoS 2/C was coated by carbon nanosheets that can provide enhanced structural stability.
10
4. Conclusion In summary, we have demonstrated a novel strategy for the synthesis of the nitrogen doped urchin-like MoS2/C by using Mo-MOFs as precursors. This material provides good conductivity and the ability to compromise the volume change during discharge and charge. As applied to SIBs, the as-prepared MoS2/C anode exhibits amazing electrochemical performance. Typically, the electrode delivers a high initial discharge capacity of 972 mA h g 1
at 100 mA g-1. When cycled at a high current density of 2000 mA g -1, it can achieve a low
capacity fading rate (0.036% per cycle) for 5000 cycles. As a result, the N-doped urchin-like MoS2/C anode may be a promising candidate for long-life sodium-ion batteries.
Acknowledgements This work was supported by National High Technology Research and Development Program of China (863 Program) (Grant no. 2013AA110106), National Natural Science Foundation of China (Grant no. 51374255 and 51572299) and the Fundamental Research Funds for the Central Universities of Central South University (Grant no. 2017zzts004).
References [1] W. Ren, Z. Zheng, C. Xu, C. Niu, Q. Wei, Q. An, K. Zhao, M. Yan, M. Qin, L. Mai, Selfsacrificed synthesis of three-dimensional Na3V2(PO4)3 nanofiber network for high-rate sodium-ion full batteries, Nano Energy 25 (2016) 145-153. [2] Y. Xu, Q. Wei, C. Xu, Q. Li, Q. An, P. Zhang, J. Sheng, L. Zhou, L. Mai, Layer-by-layer Na3V2(PO4)3 embedded in reduced graphene oxide as superior rate and ultralong-life sodiumion battery cathode, Adv. Energy Mater. 6 (2016) 1600389. [3] S. Yuan, Y.B. Liu, D. Xu, D.L. Ma, S. Wang, X.H. Yang, Z.Y. Cao, X.B. Zhang, Pure single-crystalline Na1.1V3O7.9 nanobelts as superior cathode materials for rechargeable sodium-ion batteries, Adv. Sci. 2 (2015) 1400018. [4] F. Xiong, S. Tan, Q. Wei, G. Zhang, J. Sheng, Q. An, L. Mai, Three-dimensional graphene frameworks wrapped Li3V2(PO4)3 with reversible topotactic sodium-ion storage, Nano Energy 11
32 (2017) 347-352. [5] V. Palomares, P. Serras, I. Villaluenga, K.B. Hueso, J. Carretero-Gonzalez, T. Rojo, Na-ion batteries, recent advances and present challenges to become low cost energy storage systems, Energy Environ. Sci. 5 (2012) 5884-5901. [6] S.W. Kim, D.H. Seo, X. Ma, G. Ceder, K. Kang, Electrode materials for rechargeable sodium-ion batteries: potential alternatives to current lithium-ion batteries, Adv. Energy Mater. 2 (2012) 710-721. [7] Y.F. Dong, S. Li, K.N. Zhao, C.H. Han, W. Chen, B.L. Wang, L. Wang, B.A. Xu, Q.L. Wei, L. Zhang, X. Xu, L.Q. Mai, Hierarchical zigzag Na1.25V3O8 nanowires with topotactically encoded superior performance for sodium-ion battery cathodes, Energy Environ. Sci. 8 (2015) 1267-1275. [8] Z.T. Shi, W. Kang, J. Xu, Y.W. Sun, M. Jiang, T.W. Ng, H.T. Xue, D.Y.W. Yu, W. Zhang, C.-S. Lee, Hierarchical nanotubes assembled from MoS 2-carbon monolayer sandwiched superstructure nanosheets for high-performance sodium ion batteries, Nano Energy 22 (2016) 27-37. [9] Q.Z. Yan, Y. Lu , N. Zhang , K.X. Lei , F.J. Li , J. Chen, Facile spraying synthesis and high-performance sodium storage of mesoporous MoS2/C microspheres, Adv. Funct. Mater. 26 (2016) 911-918. [10] X. Hu, W. Zhang, X. Liu, Y. Mei, Y. Huang, Nanostructured Mo-based electrode materials for electrochemical energy storage, Chem. Soc. Rev. 44 (2015) 2376-2404. [11] J. Zhou, G. Fang, A. Pan, S. Liang, Oxygen-incorporated MoS2 nanosheets with expanded interlayers for hydrogen evolution reaction and pseudocapacitor applications, ACS Appl. Mater. Inter. 8 (2016) 33681-33689. [12] M. Chhowalla, H.S. Shin, G. Eda, L.J. Li, K.P. Loh, H. Zhang, The chemistry of twodimensional layered transition metal dichalcogenide nanosheets, Nat. Chem. 5 (2013) 263275. [13] H. Jiang, D. Ren, H. Wang, Y. Hu, S. Guo, H. Yuan, P. Hu, L. Zhang, C. Li, 2D monolayer MoS2-carbon interoverlapped superstructure: engineering ideal atomic interface for lithium ion storage, Adv. Mater. 27 (2015) 3687-3695. [14] J. Wang, C. Luo, T. Gao, A. Langrock, A.C. Mignerey, C. Wang, An advanced MoS2/carbon anode for high-performance sodium-ion batteries, Small 11 (2015) 473-481. [15] X. Xiong, W. Luo, X. Hu, C. Chen, L. Qie, D. Hou, Y. Huang, Flexible membranes of MoS2/C nanofibers by electrospinning as binder-free anodes for high-performance sodium-ion batteries, Sci. Rep. 5 (2015) 9254. 12
[16] C. Zhu, X. Mu, P.A. van Aken, Y. Yu, J. Maier, Single-layered ultrasmall nanoplates of MoS2 embedded in carbon nanofibers with excellent electrochemical performance for lithium and sodium storage, Angew. Chem. Int. Ed. 53 (2014) 2152-2156. [17] S.H. Choi, Y.N. Ko, J.K. Lee, Y.C. Kang, 3D MoS 2-graphene microspheres consisting of multiple nanospheres with superior sodium ion storage properties, Adv. Funct. Mater. 25 (2015) 1780-1788. [18] L. David, R. Bhandavat, G. Singh, MoS 2/graphene composite paper for sodium-ion battery electrodes, ACS Nano 8 (2014) 1759-1770. [19] X. Xie, Z. Ao, D. Su, J. Zhang, G. Wang, MoS2/graphene composite anodes with enhanced performance for sodium-ion batteries: the role of the two-dimensional heterointerface, Adv. Funct. Mater. 25 (2015) 1393-1403. [20] L. Oakes, R. Carter, T. Hanken, A.P. Cohn, K. Share, B. Schmidt, C.L. Pint, Interface strain in vertically stacked two-dimensional heterostructured carbon-MoS2 nanosheets controls electrochemical reactivity, Nat. Commun. 7 (2016) 11796. [21] K. Pramoda, U. Gupta, I. Ahmad, R. Kumar, C.N.R. Rao, Assemblies of covalently cross-linked nanosheets of MoS2 and of MoS2–RGO: synthesis and novel properties, J. Mater. Chem. A 4 (2016) 8989-8994. [22] X. Xu, D. Yu, H. Zhou, L. Zhang, C. Xiao, C. Guo, S. Guo, S. Ding, MoS 2 nanosheets grown on amorphous carbon nanotubes for enhanced sodium storage, J. Mater. Chem. A 4 (2016) 4375-4379. [23] H. Deng, S. Grunder, K.E. Cordova, C. Valente, H. Furukawa, M. Hmadeh, F. Gándara, A.C. Whalley, Z. Liu, S. Asahina, H. Kazumori, M. O’Keeffe, O. Terasaki, J.F. Stoddart, O.M. Yaghi, Large-pore apertures in a series of metal-organic frameworks, Science 336 (2012) 1018-1023. [24] H. Hayashi, A.P. Cote, H. Furukawa, M. O/'Keeffe, O.M. Yaghi, Zeolite A imidazolate frameworks, Nat. Mater. 6 (2007) 501-506. [25] W. Lu, Z. Wei, Z.Y. Gu, T.F. Liu, J. Park, J. Park, J. Tian, M. Zhang, Q. Zhang, T. Gentle Iii, M. Bosch, H.-C. Zhou, Tuning the structure and function of metal-organic frameworks via linker design, Chem. Soc. Rev. 43 (2014) 5561-5593. [26] N. Stock, S. Biswas, Synthesis of metal-organic frameworks (MOFs): routes to various MOF topologies, morphologies, and composites, Chem. Rev. 112 (2012) 933-969. [27] Y. Zhao, Z. Song, X. Li, Q. Sun, N. Cheng, S. Lawes, X. Sun, Metal organic frameworks for energy storage and conversion, Energy Storage Mater. 2 (2016) 35-62. [28] D. Ji, H. Zhou, Y. Tong, J. Wang, M. Zhu, T. Chen, A. Yuan, Facile fabrication of MOF13
derived octahedral CuO wrapped 3D graphene network as binder-free anode for high performance lithium-ion batteries, Chem. Eng. J. 313 (2017) 1623-1632. [29] B. Liu, H. Shioyama, T. Akita, Q. Xu, Metal-organic framework as a template for porous carbon synthesis, J. Am. Chem. Soc. 130 (2008) 5390-5391. [30] F. Zheng, Y. Yang, Q. Chen, High lithium anodic performance of highly nitrogen-doped porous carbon prepared from a metal-organic framework, Nat. Commun. 5 (2014) 5261. [31] Y. Zhang, A. Pan, Y. Wang, W. Wei, Y. Su, J. Hu, G. Cao, S. Liang, Dodecahedron-shaped porous vanadium oxide and carbon composite for high-rate lithium ion batteries, ACS Appl. Mater. Inter. 8 (2016) 17303-17311. [32] A.J. Amali, J.K. Sun, Q. Xu, From assembled metal-organic framework nanoparticles to hierarchically porous carbon for electrochemical energy storage, Chem. Commun. 50 (2014) 1519-1522. [33] H. Wang, Q.L. Zhu, R. Zou, Q. Xu, Metal-organic frameworks for energy applications, Chem 2 (2017) 52-80. [34] Q.L. Zhu, W. Xia, T. Akita, R. Zou, Q. Xu, Metal-organic framework-derived honeycomb-like open porous nanostructures as precious-metal-free catalysts for highly efficient oxygen electroreduction, Adv. Mater. 28 (2016) 6391-6398. [35] G. Fang, J. Zhou, C. Liang, A. Pan, C. Zhang, Y. Tang, X. Tan, J. Liu, S. Liang, MOFs nanosheets derived porous metal oxide-coated three-dimensional substrates for lithium-ion battery applications, Nano Energy 26 (2016) 57-65. [36] J. Huang, G. Fang, K. Liu, J. Zhou, X. Tang, K. Cai, S. Liang, Controllable synthesis of highly uniform cuboid-shape MOFs and their derivatives for lithium-ion battery and photocatalysis applications, Chem. Eng. J. 322 (2017) 281-292. [37] G.Z. Fang, J. Zhou, Y.S. Cai, S.N. Liu, X.P. Tan, A.Q. Pan, S.Q. Liang, Metal-organic frameworks templated two dimensional hybrid bimetallic metal oxides with enhanced lithium/sodium storage capability, J. Mater. Chem. A, DOI: 10.1039/C7TA01961K. [38] P. Martin-Zarza, J.M. Arrieta, M. Carmen MuAoz-RocaC, P. Gili, Synthesis and characterization of new octamolybdates containing imidazole, 1-Methyl- or 2-Methylimidazole Co-ordinatively bound to molybdenum, J. Chem. Soc. Dalton Trans. (1993) 15511557. [39] X. Cao, B. Zheng, W. Shi, J. Yang, Z. Fan, Z. Luo, X. Rui, B. Chen, Q. Yan, H. Zhang, Reduced graphene oxide-wrapped MoO3 composites prepared by using metal-organic frameworks as precursor for all-solid-state flexible supercapacitors, Adv. Mater. 27 (2015) 4695-4701. 14
[40] Z. Hu, L. Wang, K. Zhang, J. Wang, F. Cheng, Z. Tao, J. Chen, MoS 2 nanoflowers with expanded interlayers as high-performance anodes for sodium-ion batteries, Angew. Chem. Int. Ed. 126 (2014) 13008-13012. [41] A. Zak, Y. Feldman, V. Lyakhovitskaya, G. Leitus, R. Popovitz-Biro, E. Wachtel, H. Cohen, S. Reich, R. Tenne, Alkali metal intercalated fullerene-like MS2 (M = W, Mo) nanoparticles and their properties, J. Am. Chem. Soc. 124 (2002) 4747-4758. [42] D.W. Su, S.X. Dou, G.X. Wang, Hierarchical orthorhombic V2O5 hollow nanospheres as high performance cathode materials for sodium-ion batteries, J. Mater. Chem. A 2 (2014) 11185-11194. [43] X. Fan, P. Xu, D. Zhou, Y. Sun, Y.C. Li, M.A.T. Nguyen, M. Terrones, T.E. Mallouk, Fast and efficient preparation of exfoliated 2H MoS2 nanosheets by sonication-assisted lithium intercalation and infrared laser-induced 1T to 2H phase reversion, Nano Lett. 15 (2015) 59565960. [44] J. Shao, T. Gao, Q. Qu, Q. Shi, Z. Zuo, H. Zheng, Ultrafast Li-storage of MoS2 nanosheets grown on metal-organic framework-derived microporous nitrogen-doped carbon dodecahedrons, J. Power Sources 324 (2016) 1-7. [45] M.A. Bissett, I.A. Kinloch, R.A.W. Dryfe, Characterization of MoS 2–graphene composites for high-performance coin cell supercapacitors, ACS Appl. Mater. Inter. 7 (2015) 17388-17398. [46] M. Sevilla, A.B. Fuertes, The production of carbon materials by hydrothermal carbonization of cellulose, Carbon 47 (2009) 2281-2289. [47] L. Lai, G. Huang, X. Wang, J. Weng, Solvothermal syntheses of hollow carbon microspheres modified with –NH2 and –OH groups in one-step process, Carbon 48 (2010) 3145-3156. [48] S. Dong, X. Chen, K. Zhang, L. Gu, L. Zhang, X. Zhou, L. Li, Z. Liu, P. Han, H. Xu, J. Yao, C. Zhang, X. Zhang, C. Shang, G. Cui, L. Chen, Molybdenum nitride based hybrid cathode for rechargeable lithium-O2 batteries, Chem. Commun. 47 (2011) 11291-11293. [49] Y.H. Lai, C.T. Yeh, J.M. Hwang, H.L. Hwang, C.T. Chen, W.H. Hung, Sputtering and Eetching of GaN surfaces, The J. Phys. Chem. B 105 (2001) 10029-10036.. [50] J.M. Jeong, K.G. Lee, S.J. Chang, J.W. Kim, Y.K. Han, S.J. Lee, B.G. Choi, Ultrathin sandwich-like MoS2@N-doped carbon nanosheets for anodes of lithium ion batteries, Nanoscale 7 (2015) 324-329. [51] Y. Qiu, W. Li, W. Zhao, G. Li, Y. Hou, M. Liu, L. Zhou, F. Ye, H. Li, Z. Wei, S. Yang, W. Duan, Y. Ye, J. Guo, Y. Zhang, High-rate, ultralong cycle-life lithium/sulfur batteries enabled 15
by nitrogen-doped graphene, Nano Lett. 14 (2014) 4821-4827. [52] S. Xiao, S. Liu, J. Zhang, Y. Wang, Polyurethane-derived N-doped porous carbon with interconnected sheet-like structure as polysulfide reservoir for lithium–sulfur batteries, J. Power Sources 293 (2015) 119-126. [53] L. Fu, K. Tang, K. Song, P.A. van Aken, Y. Yu, J. Maier, Nitrogen doped porous carbon fibres as anode materials for sodium ion batteries with excellent rate performance, Nanoscale 6 (2014) 1384-1389. [54] Z. Wang, L. Qie, L. Yuan, W. Zhang, X. Hu, Y. Huang, Functionalized N-doped interconnected carbon nanofibers as an anode material for sodium-ion storage with excellent performance, Carbon 55 (2013) 328-334. [55] Y. Xia, B. Wang, X. Zhao, G. Wang, H. Wang, Core-shell composite of hierarchical MoS2 nanosheets supported on graphitized hollow carbon microspheres for high performance lithium-ion batteries, Electrochim. Acta 187 (2016) 55-64. [56] D. Li, Y. Gong, Y. Zhang, C. Luo, W. Li, Q. Fu, C. Pan, Facile synthesis of carbon nanosphere/NiCo2O4 core-shell sub-microspheres for high performance supercapacitor, Sci. Rep-UK 5 (2015) 12903. [57] X. Wang, X. Shen, Z. Wang, R. Yu, L. Chen, Atomic-scale clarification of structural transition of MoS2 upon sodium intercalation, ACS Nano 8 (2014) 11394-11400. [58] Y.X. Wang, S.L. Chou, D. Wexler, H.K. Liu, S.X. Dou, High-performance sodium-ion batteries and sodium-ion pseudocapacitors based on MoS2/graphene composites, Chem-Eur J. 20 (2014) 9607-9612. [59] J. Xiang, D. Dong, F. Wen, J. Zhao, X. Zhang, L. Wang, Z. Liu, Microwave synthesized self-standing electrode of MoS2 nanosheets assembled on graphene foam for highperformance Li-ion and Na-ion batteries, J. Alloy. Compd. 660 (2016) 11-16. [60] S. Zhang, X. Yu, H. Yu, Y. Chen, P. Gao, C. Li, C. Zhu, Growth of ultrathin MoS 2 nanosheets with expanded spacing of (002) plane on carbon nanotubes for high-performance sodium-ion battery anodes, ACS Appl. Mater. Inter. 6 (2014) 21880-21885. [61] D. Su, S. Dou, G. Wang, Ultrathin MoS2 nanosheets as anode materials for sodium-ion batteries with superior performance, Adv. Energy Mater. 5 (2015) 1401205.
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Figures and captions
Scheme 1 Illustration of preparation process of N doped urchin-like MoS2/C.
Fig. 1. XRD pattern of (a) Mo-MOFs, (b) as-prepared MoS2/C and annealed MoS2/C.
17
Fig. 2 (a-b) TEM images and (c-d) HRTEM image of the N doped urchin-like MoS2/C.
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Fig. 3. (a) N2 adsorption/desorption isotherm and (b) corresponding BJH pore-size distribution curve of annealed MoS2/C.
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Fig. 4 Core level XPS spectrum of annealed MoS2/C.
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Fig. 5 FESEM images at various sulfuration time: (a) fresh Mo-MOFs, (b) 1.5 hour, (c) 3 hour and (d) 12 hour.
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Fig. 6 Electrochemical performance of N-doped MoS2/C in SIBs, in comparison with the pure MoS2: (a) the first three CV curves at a scan rate of 0.1 mV s -1, (b) the charge/discharge profiles at 100 mA g-1, (c) the cycling performance and corresponding coulombic efficiency at 100 mA g-1, (d) the rate performance at various current density from 100 to 5000 mA g -1, (e) the high-rate (2000 mA g-1) cycling performance and corresponding coulombic efficiency.
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Highlights N-doped MoS2/C is successfully synthesized by using Mo-MOFs as the precursors. The formation mechanism of the hollow MoS2/C nanospheres is proposed. The MoS2 anode demonstrates superior long-term cycling performance for SIBs.
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